Because one Amibroker Instance cannot open two databases at the same time,
So the solution I think of is to open two Amibroker Instance , each open a database, and then exchange data between the two instance .
There are any built-in function that can exchange data between two Amibroker instance
You should avoid such situation (having to talk between instances). The best idea is to keep things separate if they are separate or to just use single instance and one database if you want to have easy "communication".
Having said that if you really must, you can exchange data between applications in a number of ways but the easiest is just plain text files (fopen/fgets/fputs/fclose).